1. Field of the Invention
This invention relates to a positive displacement pump for introduction and delivery of a subject liquid by repeatedly pressing a cylindrical hollow member containing the subject liquid in a direction normal to a diametric line of the hollow member into a flat or triangular shape in its radial cross section. Stresses on the hollow member due to the repeated depression and restoration are imposed symmetrically to prolong the life of the hollow member.
2. Prior Art
The density of semiconductor integrated circuits has been, and will soon be, made higher and higher from 16 MB to 64 MB, and further to 1 GB. Accordingly, more strict standards are requested in terms of allowance for impurities in semiconductors for making ultra-high-density integrated circuits.
For example, in case of chemical liquids used in the manufacturing process of semiconductors, particles of 0.1 mm in a chemical liquid should not exceed 20 per 1 cm.sup.3, and particles of 0.05 mm should not exceed 10 per 1 cm.sup.3.
In general, a filter is used to remove particles from a chemical liquid, and filtration using a filter needs a pressurizing pump with a certain performance.
Taking requirements on resistance to a high pressure and resistance to chemicals into account, recent technologies for manufacturing semiconductors use diaphragm pumps, bellows pumps, or the like, whose major parts are made of polytetrafluoroethylene (PTFE), which is a four-fluorine-contained polymer, together with filters for removing impurities from subject liquids.
These conventional pumps, however, involve the following problems. First, resistance to high temperatures and high pressures is insufficient for use in fabrication of semiconductors.
It is known that there is a certain relationship between the pressure of a subject liquid and the square of pores of a filter. Namely, according to the Hagen-Poiseuille law, if the square of pores of a filter is reduced to 1/2, the pressure of a subject liquid must be quadrupled to ensure the same flow amount.
For the reason mentioned above, filters used in fabrication of semiconductors are requested to have a pore size of 0.1 mm to 0.05 mm.
If a filter with the pore size of 0.1 mm is replaced by a filer with the pore size of 0.05 mm, the pump pressure must be raised by a factor of sixteen. Actually, however, the maximum outlet pressure of conventional pumps is 2 to 5 kg/cm.sup.2 at 20.degree. C. and 1 kg/cm.sup.2 at 150.degree. C., and pumps are operated at the maximum pressure. Thus, it is impossible to rely on any further increase in outlet pressure.
Therefore, when the square of pores of a filter is 1/2, units each containing a pump and a filter must be increased by a factor of sixteen to ensure the same filter outlet gain. An alternative way of increasing the filter-through efficiency is to increases the temperature of the subject liquid and decrease the viscosity of the liquid. However, at temperatures beyond 120.degree. C., conventional pumps often loose their pumping function due to a thermal deformation of bellows, or other elements, caused by a decrease in rigidity.
A second problem with conventional pumps is that both diaphragm pumps and bellows pumps rely on deformation of diaphragms or bellows made of a plastic resin for suction and discharge of a subject liquid, and deformation of diaphragms or bellows always occurs at particular portions thereof. Therefore, stress cracking is liable to occur due to mechanical fatigue of their materials caused by repetitive bending motion and concentration of a high stress, and this can damage the pumps. To cope with the problem, conventional pumps are equipped with a leakage sensor for detecting leakage of a subject liquid. However, when the sensor detects leakage of the liquid, the pump has already been destroyed.